Data race when use http client

[domashevskyi]

Reproduced on:

• SDK version: V1.0.0-rc.8.
• GO version: go1.11.1 linux/amd64

When build with - race flag, data race warning occurs upon sending http request:

WARNING: DATA RACE
Read at 0x00c000a201b0 by goroutine 56:
vendor/github.com/aws/aws-xray-sdk-go/xray.(*HTTPSubsegments).GotFirstResponseByte()
vendor/github.com/aws/aws-xray-sdk-go/xray/httptrace.go:159 +0x47
privacyapi/vendor/github.com/aws/aws-xray-sdk-go/xray.NewClientTrace.func10()
vendor/github.com/aws/aws-xray-sdk-go/xray/httptrace.go:212 +0x41
net/http.http2traceFirstResponseByte()
/usr/local/go/src/net/http/h2_bundle.go:3035 +0x7d
net/http.(*http2clientConnReadLoop).processHeaders()
/usr/local/go/src/net/http/h2_bundle.go:8360 +0x52a
net/http.(*http2clientConnReadLoop).run()
/usr/local/go/src/net/http/h2_bundle.go:8292 +0x87d
net/http.(*http2ClientConn).readLoop()
/usr/local/go/src/net/http/h2_bundle.go:8189 +0x8c

Previous write at 0x00c000a201b0 by main goroutine:
vendor/github.com/aws/aws-xray-sdk-go/xray.(*HTTPSubsegments).WroteRequest()
vendor/github.com/aws/aws-xray-sdk-go/xray/httptrace.go:152 +0x156
vendor/github.com/aws/aws-xray-sdk-go/xray.NewClientTrace.func9()
vendor/github.com/aws/aws-xray-sdk-go/xray/httptrace.go:209 +0x55
net/http.http2traceWroteRequest()

[luluzhao]

@domashevskiy , sorry for the late response. I merged @t-yuki PR and will make sure bring this fix to next release.