feat: set content-type only when it doesn't exist on user custom headers

[simpleapples]

Hi @guonaihong

When I use gout to send a JSON like the code below.

err = g.POST(url)
    .SetHeader(core.H{"Content-Type": "application/json"})
    .SetJSON("{\"key\":\"val\"}")
    .Do()

I found that the http headers gout sent likes this

Content-Type: "application/json,application/json"

Then I investigated the source code and found that on this line

gout/dataflow/req.go

Line 331 in 511f37c

r.addContextType(req)

gout adds Content-Type to headers whehter it exists or not. Which means if the user uses SetHeader('Content-Type', 'xxx') and SetJSON() in the same request. The Content-Type will have two values. Because the Add func called by SetJSON and SetHeader appends values to a key, not replace it. As the screenshot shows.

To avoid this situation. I believe we should check existence of Content-Type before setting it. And if the Content-Type exists, means users already set it by themeselves (for better visability or other purposes) or users want to use a different header value. gout shouldn't set it or replace it.