All the logic is based on the HTML_ESCAPE constant available in
ERB::Util, so it seems more logic to have the entire method there and
just delegate the helper to use it.

        length      user     system      total        real
before  6      0.010000   0.000000   0.010000 (  0.012378)
after   6      0.010000   0.000000   0.010000 (  0.012866)
before  60     0.040000   0.000000   0.040000 (  0.046273)
after   60     0.040000   0.000000   0.040000 (  0.036421)
before  600    0.390000   0.000000   0.390000 (  0.390670)
after   600    0.210000   0.000000   0.210000 (  0.209094)
before  6000   3.750000   0.000000   3.750000 (  3.751008)
after   6000   1.860000   0.000000   1.860000 (  1.857901)

Revert html_escape to do a single gsub again, but add the "n" flag (no
language, i.e. not multi-byte) to protect against XSS via invalid utf8
Signed-off-by: NJosé Valim <jose.valim@gmail.com>

revert the changes from c60995f3 - related to marking sub,gsub as unavailable to use with safe strings

"Safe Buffer" should either be the constant with the class name,
or go in lower case. I've chosen to follow the same terminology
that is used in the AS core extensiong guide, "safe string",
though "safe buffer" is also used elsewhere, we should pick one.

Also make sure that the versions of those methods which modify a string in place such as gsub! can't be called on safe buffers at all.

Conflicts:

	activesupport/test/safe_buffer_test.rb

Several AP tests fail after this change.

This reverts commit aaf01cd5, reversing
changes made to 9cc18c52.

The author of ERB sais, his eRuby implementation was originally named "ERb/ERbLight" and then renamed to "ERB" when started bundled as a Ruby standard lib.
http://www2a.biglobe.ne.jp/~seki/ruby/erb.html