Fix concurrent write bug in reader.Render

[halvfigur]

If a caller passed in a map retained within the caller's context as extraHeaders to gin.Context.DataFromReader() then a race to write the "Content-Lenght" header would occur.

// globalHeader is passed to gin.Context.DataFromReader var globalHeaders = map[string]string{ "cache-control": "public, max-age=3600", }

func (c *gin.Context) { //...

// DataFromReader must not write to globalHeaders
c.DataFromReader(code, contentLength, contentType, reader,
globalHeaders)

}

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