Master - Fix "=" character in HttpPostRequestDecoder

Motivation
Issue #3004 shows that "=" character was not supported as it should in
the HttpPostRequestDecoder in form-data boundary.

Modifications:
Add 2 methods in StringUtil

splitInTwo: String split with a 2 parts only (to prevent multiple '=' to
be source of extra split while not needed)
afterDelim: String part after delimiter (since first part is not needed)
Use those methods in HttpPostRequestDecoder. Change and the
HttpPostRequestDecoderTest to check using a boundary beginning with "=".

Results:
The fix implies more stability and fix the issue.
Additionnal fix

As per request, change the name of the methods to split with maxPart arg
and to substringAfter, and relative changes in Http multipart codec.
fix style
Merge pull request #3011 from fredericBregier/master-fix--=--boundary-HttpPostRequestDecoder

Master - Fix "=" character in HttpPostRequestDecoder