Spellchecker: provide an option to pass sensitive data using POST request

[Wiktor Walc]

Right now the built-in spell checker sends the content entered by user using GET requests (at least after clicking "Finish checking"). It is causing problems for some companies, where security filters block certain URLs with specific keywords.

It would be nice to at least make it configurable (to allow user to choose POST instead of default GET for such requests), if not simply switch to POST by default.

Note that sending POST requets instead of GET would also allow spellchecker to split content into larger chunks. This is causing serious performance issues, as explained in #6119.

Sample URL:

/ckeditor/plugins/wsc/dialogs/ciframe.html?id=58745&cmd=part&data=g!2520teeth!2520you!2520have!2521!2526quot!253B!253Cbr!2520/!253E!250D!250A!2509To!2520which!2520the!2520wolf!2520replies!252C!2520!2526quot!253BThe!2520better!2520to!2520eat!2520you!2520with!252C!2526quot!253B!2520and!2520swallows!2520her!2520whole!252C!2520too.!253C/p!253E!250D!250A!253Cp!253E!250D!250A!2509A!2520!253Ca!2520href!253D!2522http!253Aen.wikipedia.org/wiki/Hunter!2522!2520title!253D!2522Hunter!2522!253Ehunter!253C/a!253E!252C!2520however!252C!2520comes!2520to!2520the!2520rescue!2520and!2520cuts!2520the!2520wolf!2520open.!2520Little!2520Red!2520Riding!2520Hood!2520and!2520her!2520grandmother!2520emerge!2520unharmed.!2520They!2520fill!2520the!2520wolf!2526!252339!253Bs!2520body!2520with!2520heavy!2520stones!252C!2520which!2520drown!2520him!2520when!2520he!2520falls!2520into!2520a!2520well.!2520Other!2520versions!2520of!2520the!2520story!2520have!2520had!2520the!2520grandmother!2520shut!2520in!2520the!2520closet!2520instead!2520of!2520eaten!252C!2520and!2520some!2520have!2520Little!2520Red!2520Riding!2520Hood!2520saved!2520by!2520the!2520hunter!2520as!2520the!2520wolf!2520advances!2520on!2520her!2520rather!2520than!2520after!2520she!2520is!2520eaten.!253C/p!253E!250D!250A!253Cp!253E!250D!250A!2509The!2520tale!2520makes!2520the!2520clearest!2520contrast!2520between!2520the!2520safe!2520world!2520of!2520the!2520village!2520and!2520the!2520dangers!2520of!2520the!2520!253Ca!2520href!253D!2522http