fix EachKey pIdxFlags allocation

[unxcepted]

Description: This pr implements patch from #223, which original author never made a PR for. Fixes one allocation in EachKey function caused by allocating pIdxFlags array.

Benchmark before change:

BenchmarkJsonParserEachKeyManualMedium
BenchmarkJsonParserEachKeyManualMedium-12         805526              6450 ns/op               8 B/op          2 allocs/op
BenchmarkJsonParserEachKeyStructMedium
BenchmarkJsonParserEachKeyStructMedium-12         644608              9243 ns/op             408 B/op         12 allocs/op

Benchmark after change:

BenchmarkJsonParserEachKeyManualMedium
BenchmarkJsonParserEachKeyManualMedium-12         947583              6521 ns/op               0 B/op          0 allocs/op
BenchmarkJsonParserEachKeyStructMedium
BenchmarkJsonParserEachKeyStructMedium-12         776686              8836 ns/op             400 B/op         10 allocs/op

[buger]

Sorry for delay, looks nice!