client: Client.buildRequest: don't set content-header if not set

This function was setting `text/plain` as default content-type for any
request that had a non-nil body.

However, this would also set the content-type if (e.g.) `http.NoBody` was set,
or if an empty reader was used, which would result in the daemon potentialy
rejecting the request, as it validates request to be using `application/json`;
d9ee22d1ab/daemon/server/httputils/httputils.go (L47-L58)

    === RUN   TestCommitInheritsEnv
        commit_test.go:30: assertion failed: error is not nil: Error response from daemon: unsupported Content-Type header (text/plain): must be 'application/json'
    --- FAIL: TestCommitInheritsEnv (0.02s)

This patch removes setting the default content-type.

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

client/request.go

@@ -110,9 +110,6 @@ func (cli *Client) buildRequest(ctx context.Context, method, path string, body i
 		req.Host = DummyHost
 	}
 
-	if body != nil && req.Header.Get("Content-Type") == "" {
-		req.Header.Set("Content-Type", "text/plain")
-	}
 	return req, nil
 }