Fix parameter value retrieval

Initial implementation of MutableHttpRequest restricts all parameters have a single value. Change is to make the behavior consistent with http protocol which allows multiple (array) value for all parameters.

Difference from initial patch made to stinger 2.2.2:
 - this version does NOT make use of the debug flag introduced in the aforementioned patch
 - this version does NOT touch the headers
 - this version does NOT check on possible malformed URI, change does not affect the goal of this patch
 - this version adjusts Encode such that the process iterates over the entire array instead of processing only the first value.