Handle prometheus post query cache correctly

[wu0407]

1. get URL query params from Post request, if no request body then fall back to get URL query params.
2. Support request content type is application JSON with charset

#587

Signed-off-by: xiaoqing xiaoqingnb@gmail.com

[wu0407]

@jranson @crandles

[coveralls]

Pull Request Test Coverage Report for Build 7073583450

• 7 of 7 (100.0%) changed or added relevant lines in 2 files are covered.
• 10 unchanged lines in 1 file lost coverage.
• Overall coverage decreased (-0.06%) to 92.275%

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Files with Coverage Reduction	New Missed Lines	%
pkg/proxy/params/params.go	10	73.33%

Totals
Change from base Build 7073416636:	-0.06%
Covered Lines:	17284
Relevant Lines:	18731

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💛 - Coveralls