Add support for multimaps in query strings

[betabandido]

Currently, Gin supports parsing maps in query strings using GetQueryMap. The current implementation, however, is limited to simple maps where values are strings. A simple change to the code enables parsing more complex maps where values are string arrays. For instance, the following URL contains such a map:

/test?map[key1]=value11&map[key1]=value12&map[key2]=value2

We have use-cases where we need to parse such URLs. Given the change is relatively minor, and it is backwards compatible, it would be nice to have this feature added.

I chose multimap to refer to a map containing string arrays (see https://en.wikipedia.org/wiki/Multimap). But, there might be a better name.

I will document the new methods in the README if you think it is a feature you want to add to Gin.

I decided not to duplicate the get method to reduce code duplication. The tradeoff is a slight performance reduction. I did some performance evaluation and the results were:

Original GetQueryMap: 216 ns / call
Modified GetQueryMap: 419 ns / call

I believe this is a negligible increase in performance, but you should have a better idea on the latency breakdown across the whole lifespan of a process request. So, let me know if you think it is better not to pay the extra latency.

[codecov]

Codecov Report

Merging #2139 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #2139      +/-   ##
==========================================
+ Coverage   98.74%   98.75%   +<.01%     
==========================================
  Files          40       40              
  Lines        2236     2248      +12     
==========================================
+ Hits         2208     2220      +12     
  Misses         14       14              
  Partials       14       14

Impacted Files	Coverage Δ
context.go	98.69% <100%> (+0.03%)	⬆️

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[appleboy]

Please fix the conflicts and move to 1.11