Fix set header to JSON earlier in GraphQL response

Update the GraphQL handler to set the Response Header to JSON earlier for
error messages to be returned as JSON and not text/html.

Fixes #519

== Notes:
- Add checks for JSON Content-Type checks in decode bad queries tests

handler/graphql.go

@@ -299,6 +299,7 @@ func (gh *graphqlHandler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
+	w.Header().Set("Content-Type", "application/json")
 	var reqParams params
 	switch r.Method {
 	case http.MethodGet:
@@ -320,7 +321,6 @@ func (gh *graphqlHandler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 		w.WriteHeader(http.StatusMethodNotAllowed)
 		return
 	}
-	w.Header().Set("Content-Type", "application/json")
 
 	ctx := r.Context()
 

handler/graphql_test.go

@@ -45,30 +45,35 @@ func TestHandlerPOST(t *testing.T) {
 	t.Run("decode failure", func(t *testing.T) {
 		resp := doRequest(h, "POST", "/graphql", "notjson")
 		assert.Equal(t, http.StatusBadRequest, resp.Code)
+		assert.Equal(t, resp.HeaderMap.Get("Content-Type"), "application/json")
 		assert.Equal(t, `{"errors":[{"message":"json body could not be decoded: invalid character 'o' in literal null (expecting 'u')"}],"data":null}`, resp.Body.String())
 	})
 
 	t.Run("parse failure", func(t *testing.T) {
 		resp := doRequest(h, "POST", "/graphql", `{"query": "!"}`)
 		assert.Equal(t, http.StatusUnprocessableEntity, resp.Code)
+		assert.Equal(t, resp.HeaderMap.Get("Content-Type"), "application/json")
 		assert.Equal(t, `{"errors":[{"message":"Unexpected !","locations":[{"line":1,"column":1}]}],"data":null}`, resp.Body.String())
 	})
 
 	t.Run("validation failure", func(t *testing.T) {
 		resp := doRequest(h, "POST", "/graphql", `{"query": "{ me { title }}"}`)
 		assert.Equal(t, http.StatusUnprocessableEntity, resp.Code)
+		assert.Equal(t, resp.HeaderMap.Get("Content-Type"), "application/json")
 		assert.Equal(t, `{"errors":[{"message":"Cannot query field \"title\" on type \"User\".","locations":[{"line":1,"column":8}]}],"data":null}`, resp.Body.String())
 	})
 
 	t.Run("invalid variable", func(t *testing.T) {
 		resp := doRequest(h, "POST", "/graphql", `{"query": "query($id:Int!){user(id:$id){name}}","variables":{"id":false}}`)
 		assert.Equal(t, http.StatusUnprocessableEntity, resp.Code)
+		assert.Equal(t, resp.HeaderMap.Get("Content-Type"), "application/json")
 		assert.Equal(t, `{"errors":[{"message":"cannot use bool as Int","path":["variable","id"]}],"data":null}`, resp.Body.String())
 	})
 
 	t.Run("execution failure", func(t *testing.T) {
 		resp := doRequest(h, "POST", "/graphql", `{"query": "mutation { me { name } }"}`)
 		assert.Equal(t, http.StatusOK, resp.Code)
+		assert.Equal(t, resp.HeaderMap.Get("Content-Type"), "application/json")
 		assert.Equal(t, `{"errors":[{"message":"mutations are not supported"}],"data":null}`, resp.Body.String())
 	})
 }