core: allow URL parameters and POST bodies to co-exist.

Node 8.14.0 prohibits HTTP headers that exceed 8 KB (source:
https://nodejs.org/en/blog/vulnerability/november-2018-security-releases/#denial-of-service-with-large-http-headers-cve-2018-12121).

This patch allows for the parameters within the body of an HTTP POST request to
be used in addition to those within the URL (and will override them).

Closes #3568.

---
Muxator 2019-10-19:
- this commit was cherry-picked from 882b93487f
- it was modified to include the necessary changes in the documentation

tests/backend/specs/api/pad.js

@@ -387,7 +387,8 @@ describe('createPad', function(){
 
 describe('setText', function(){
   it('Sets text on a pad Id', function(done) {
-    api.get(endPoint('setText')+"&padID="+testPadId+"&text="+text)
+    api.post(endPoint('setText')+"&padID="+testPadId)
+    .send({text: text})
     .expect(function(res){
       if(res.body.code !== 0) throw new Error("Pad Set Text failed")
     })
@@ -410,7 +411,8 @@ describe('getText', function(){
 
 describe('setText', function(){
   it('Sets text on a pad Id including an explicit newline', function(done) {
-    api.get(endPoint('setText')+"&padID="+testPadId+"&text="+text+'%0A')
+    api.post(endPoint('setText')+"&padID="+testPadId)
+    .send({text: text+'\n'})
     .expect(function(res){
       if(res.body.code !== 0) throw new Error("Pad Set Text failed")
     })