Backport http post request decoder 4.1 to 4.0 + "=" Boundary fix

 Fix "=" character in HttpPostRequestDecoder
 Backport 4.1 to 4.0 on HttpPostRequestDecoder

Motivation:
4.0 was not modified in the same time than 4.1 while the difference was limited and was revealed in issue #2886 .
Issue #3004 shows that "=" character was not supported as it should in the HttpPostRequestDecoder in form-data boundary.

Modifications:
Backport of 4.1
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:
Backport done (Issue #2886 fix)
The fix implies more stability and fix the issue #3004 .