fixed null in number json tag string

[nikolaydubina]

This fixes recently reported bug, when null is passed to number with json tag set to string. As highlighted in issue, standard library will place zero value in such case (GoPlayground). However, json-iterator previously crashed with error like this:

--- FAIL: Test_unmarshal (0.00s)
    --- FAIL: Test_unmarshal/[41]{"field":_null} (0.00s)
        require.go:794: 
            	Error Trace:	value_test.go:52
            	Error:      	Received unexpected error:
            	            	Field: stringModeNumberDecoder: expect ", but found n, error found in #10 byte of ...|"field": null}|..., bigger context ...|{"field": null}|...
            	Test:       	Test_unmarshal/[41]{"field":_null}
            	Messages:   	jsoniter

Test plan:

• added test that fails without this modification
• all other tests pass

[codecov]

Codecov Report

Merging #480 into master will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #480   +/-   ##
=======================================
  Coverage   86.32%   86.33%           
=======================================
  Files          41       41           
  Lines        5112     5115    +3     
=======================================
+ Hits         4413     4416    +3     
  Misses        558      558           
  Partials      141      141           

Impacted Files	Coverage Δ
reflect_struct_decoder.go	82.75% <100.00%> (+0.06%)	⬆️

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[nikolaydubina]

cc: @taowen @AllenX2018 @thockin

[nikolaydubina]

cc: @cch123 @olegshaldybin

[AllenX2018]

Thanks for the pr. I'm sorry that I've been busy these days. I will check it in this week.

[AllenX2018]

Merged. Thanks for fixing this again.