Also add test to ensure basic model does not explode when
initialized with nil.

PATCH is the correct HTML verb to map to the #update action. The
semantics for PATCH allows for partial updates, whereas PUT requires a
complete replacement.

Changes:
* adds config.default_method_for_update you can set to :patch
* optionally use PATCH instead of PUT in resource routes and forms
* adds the #patch verb to routes to detect PATCH requests
* adds #patch? to Request
* changes documentation and comments to indicate support for PATCH

This change maintains complete backwards compatibility by keeping :put
as the default for config.default_method_for_update.

There is an example in Rails documentation that suggests implementing
assign_attributes method for ActiveModel interface, that by default
sends option role with nil. Since mass_assignment_authorizer never
is called without args, we can move the default value internally.

Serialization uses only the attributes hash's keys and calls
methods that are of the same name as the keys on the serialized
object.

Allow infinite values for validates_length_of. Particularly useful
for prettily defining an open ended range such as

validates_length_of :human_stupidity, :within => 0..Float::INFINITY

message quote fix

Since ActiveModel::Errors instance keeps all error messages as hash
we should duplicate this object as well.

Previously ActiveModel::Errors was a subclass of ActiveSupport::OrderedHash,
which results in different behavior on `dup`, this may result in regression for
people relying on it.

Without that patch when using ActiveModel::AttributeMethods
in a class that does not respond to `attributes` method,
stack level too deep error will be raised on non existing
method. While documentation is clear that you need to define
`attributes` method in order to use AttributeMethods module,
`stack level too deep` is rather obscure and hard to debug,
therefore we should try to not break `method_missing` if
someone forgets about defining `attributes`.

Fixed failing tests
This reverts commit 8350ce97.

This reverts commit 675094b2.