Send correct content-type when creating bucket

[iyigunform3]

Bucket creation request must have the application/json content-type according to the runscope documentation. This content type is being enforced now and providing application/x-www-form-urlencoded causes a 415 Unsupported Media Type response.

Before the change the tests were failing with following error:

=== RUN   TestCreateBucket
[DEBUG] creating bucket test
[DEBUG]	 	request: POST /buckets url.Values{"name":[]string{"test"}, "team_uuid":[]string{"0b648b14-4e1c-49e2-aad6-013cdf0d8f5f"}}
[DEBUG]	 &http.Request{Method:"POST", URL:(*url.URL)(0xc0004d4750), Proto:"HTTP/1.1", ProtoMajor:1, ProtoMinor:1, Header:http.Header{"Accept":[]string{"application/json"}, "Authorization":[]string{"Bearer e84c44cb-2533-4a32-814c-6657e9ce3f5f"}, "Content-Type":[]string{"application/x-www-form-urlencoded"}}, Body:io.nopCloserWriterTo{Reader:(*strings.Reader)(0xc0002bcbc0)}, GetBody:(func() (io.ReadCloser, error))(0x66ff60), ContentLength:56, TransferEncoding:[]string(nil), Close:false, Host:"api.runscope.com", Form:url.Values(nil), PostForm:url.Values(nil), MultipartForm:(*multipart.Form)(nil), Trailer:http.Header(nil), RemoteAddr:"", RequestURI:"", TLS:(*tls.ConnectionState)(nil), Cancel:(<-chan struct {})(nil), Response:(*http.Response)(nil), ctx:(*context.emptyCtx)(0xc00001a140)}
[DEBUG]	 	response: 415 {"data": {}, "meta": {"status": "error"}, "error": {"status": 415, "message": "Unsupported Media Type"}}

    bucket_test.go:15: Error creating bucket: test
--- FAIL: TestCreateBucket (0.24s)
panic: runtime error: invalid memory address or nil pointer dereference [recovered]
	panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x10 pc=0x6a434d]

goroutine 107 [running]:
testing.tRunner.func1.2({0x6e8d60, 0x96f5b0})
	/usr/local/go/src/testing/testing.go:1396 +0x24e
testing.tRunner.func1()
	/usr/local/go/src/testing/testing.go:1399 +0x39f
panic({0x6e8d60, 0x96f5b0})
	/usr/local/go/src/runtime/panic.go:884 +0x212
github.com/ewilde/go-runscope.TestCreateBucket(0xc000358d00)
	/home/iyigun/go/src/github.com/ewilde/go-runscope/bucket_test.go:18 +0x10d
testing.tRunner(0xc000358d00, 0x766de8)
	/usr/local/go/src/testing/testing.go:1446 +0x10b
created by testing.(*T).Run
	/usr/local/go/src/testing/testing.go:1493 +0x35f
FAIL	github.com/ewilde/go-runscope	23.702s
?   	github.com/ewilde/go-runscope/examples	[no test files]