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.
Could we instead have the following method:

* `String[] split(String value, char delim, int maxParts)`

if `maxParts` is 1, we can simply return `value` as is. To get what you want, you could pass 2 as `maxParts`.