Add support to json format for _stats API

[shaktals]

Changes

Adds support for API consumer to specify Accept request header as applicaton/json so that the response complies with that format.

Fixes

#508

Tests

Tasks to complete before merging PR:

• Ensure system tests are passing. If not Run them manually to check for any regressions 📋
• Do any new system tests need added to test this change? do any existing system tests need updated? If so create a PR at 0chain/system_test
• Merge your system tests PR to master AFTER merging this PR

[codecov-commenter]

Codecov Report

Merging #663 (ae880b7) into staging (88b50cb) will decrease coverage by 0.11%.
The diff coverage is 0.00%.

@@             Coverage Diff             @@
##           staging     #663      +/-   ##
===========================================
- Coverage    19.79%   19.67%   -0.12%     
===========================================
  Files           66       66              
  Lines         7482     7527      +45     
===========================================
  Hits          1481     1481              
- Misses        5774     5819      +45     
  Partials       227      227              

Flag	Coverage Δ
Unit-Tests	19.67% <0.00%> (-0.12%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
code/go/0chain.net/blobbercore/handler/handler.go	57.48% <0.00%> (-5.15%)	⬇️
...o/0chain.net/blobbercore/handler/handler_common.go	23.76% <0.00%> (-2.91%)	⬇️

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[peterlimg]

lgtm