This commit *adds* the "intercepted" headers to the ClientHttpRequest,
as opposed to replacing them, which is what happened before this commit.

Issue: SPR-15166

Issue: SPR-15024

An update on the last commit switching from:
List<Locale> getAcceptLanguageAsLocales() to
Locale getAcceptLanguageAsLocale()

This best supports the scenario for the most preferred Locale.
If there is a need to look at the prioritized list of languages it's
best to use Locale.filter with the LocaleRange's.

This is explained in the Javadoc for getAcceptLanguage().

Issue: SPR-15024

The use of Locale.LanguageRange for the Accept-Language header makes
sense as it gives the most flexibility for a client to set a weighted
list and for a server to do filtering via Locale#filter.

This commit adds an additional convenience method that turns
the LangugeRange list to a list of Locale's also filtering out a
wildcard (i.e. "*"). A List<Locale> is the most basic way to access
prefered languages and needed when filtering is not required.

Issue: SPR-15024

Issue: SPR-14536

Issue: SPR-15024

This commit introduces a new ContentDisposition class designed
to parse and generate Content-Disposition header value as defined
in RFC 2183. It supports the disposition type and the name,
filename (or filename* when encoded according to RFC 5987) and
size parameters.

This new class is usually used thanks to
HttpHeaders#getContentDisposition() and
HttpHeaders#setContentDisposition(ContentDisposition).

Issue: SPR-14408

this PR fixes the spelling error for the word 'recommendation'

(cherry picked from commit 37670924)

Issue: SPR-14635

This commit also sets the test source encoding to UTF-8.

Issue: SPR-14547

Added getHost/setHost to HttpHeaders, returning/taking a
InetSocketAddress.

This commit adds support for HTTP header field parameters encoding, as
described in RFC5987.
Note that the default implementation still relies on US-ASCII encoding,
as the latest rfc7230 Section 3.2.4 says that:

> Newly defined header fields SHOULD limit their field values to
  US-ASCII octets

Issue: SPR-14547

Includes optimized method/header resolution in CorsConfiguration.

Issue: SPR-14617

Issue: SPR-14506

Issue: SPR-13188

Issue: SPR-14406

Prior to this change, getting header values with `HttpHeaders` when
headers are multi-valued would cause issues.
For example, for a given HTTP message with headers:
    Cache-Control: public, s-maxage=50
    Cache-Control: max-age=42

Getting a `List` of all values would return <"public", "s-maxage=50">
and getting the header value would return "public, s-maxage=50".

This commit takes now into account multi-valued HTTP headers and adds
new getters/setters for "If-Match" and "If-Unmodified-Since" headers.

Note that for ETag-related headers such as "If-Match" and
"If-None-Match", a special parser has been implemented since ETag values
can contain separator characters.

Issue: SPR-14223, SPR-14228

Issue: SPR-14144

Issue: SPR-14070

Issue: SPR-13776

This commit adds CORS related headers to HttpHeaders
and update DefaultCorsProcessor implementation to
use ServerHttpRequest and ServerHttpResponse instead
of HttpServletRequest and HttpServletResponse. Usage
of ServerHttpResponse allows to avoid using Servlet 3.0
specific methods in order keep CORS support Servlet 2.5
compliant.

Issue: SPR-12885

This commit introduces support for HTTP byte ranges in the
ResourceHttpRequestHandler. This support consists of a number of
changes:

- Parsing of HTTP Range headers in HttpHeaders, using a new HttpRange
  class and inner ByteRange/SuffixByteRange subclasses.
- MIME boundary generation moved from FormHttpMessageConverter to
  MimeTypeUtils.
- writePartialContent() method introduced in ResourceHttpRequestHandler,
  handling the byte range logic
- Additional partial content tests added to
  ResourceHttpRequestHandlerTests.

Issue: SPR-10805

Issue: SPR-12578

Issue: SPR-12173

Issue: SPR-12063

(cherry picked from commit 86ea3059)

Prior to this commit, calls to getAllow would fail is setAllow was set
to an EmptyCollection right before.

    java.lang.IllegalArgumentException: No enum constant
    org.springframework.http.HttpMethod

This commit fixes this by testing the header value for an empty value
before trying to use it to get a value from the Enum.

Issue: SPR-11917

Issue: SPR-11749

After this change, AbstractSockJsService does not add CORS headers if
the response already contains an "Access-Control-Allow-Origin" header.
Essentially it backs off assuming CORS headers are handled centrally
e.g. through a Filter.

In order to support this, the ServletServerHttpResponse now returns an
instance of HttpHeaders that also provides access to headers already
present in the HttpServletResponse.

Issue: SPR-11443

Issue: SPR-11241

Fix remaining Java compiler warnings, mainly around missing
generics or deprecated code.

Also add the `-Werror` compiler option to ensure that any future
warnings will fail the build.

Issue: SPR-11064