Remove trailing slash on identity server

[NicolasBuquet]

Old Account data can have a trailing slash at the end of their Identity Server.
This can lead to unrecognized URL on the backend (like on 'invite to room') because the URL is then constructed with a double slash in its path.
This lead to error 500 for these calls.
So, fix this trailing slash as soon as we receive it.

Pull Request Checklist

• Pull request is based on the develop branch
• Pull request contains a changelog file in ./changelog.d. See https://github.com/matrix-org/matrix-ios-sdk/blob/develop/CONTRIBUTING.md#changelog
• Pull request includes a sign off

[NicolasBuquet]

Bad files commited

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 40.00000% with 3 lines in your changes missing coverage. Please review.

Project coverage is 19.90%. Comparing base (626e03f) to head (9d70a70).
Report is 178 commits behind head on develop.

Files with missing lines	Patch %	Lines
.../Data/RoomList/Common/MXRoomListDataSortable.swift	0.00%	1 Missing ⚠️
...mList/CoreData/MXCoreDataRoomListDataFetcher.swift	0.00%	1 Missing ⚠️
MatrixSDK/MXSession.m	66.66%	1 Missing ⚠️

❗ There is a different number of reports uploaded between BASE (626e03f) and HEAD (9d70a70). Click for more details.

HEAD has 1 upload less than BASE

Flag	BASE (626e03f)	HEAD (9d70a70)
	2	1

Additional details and impacted files

@@             Coverage Diff              @@
##           develop    #1897       +/-   ##
============================================
- Coverage    35.63%   19.90%   -15.74%     
============================================
  Files          617      564       -53     
  Lines        96959    79304    -17655     
  Branches     41454    33431     -8023     
============================================
- Hits         34552    15784    -18768     
- Misses       61487    63504     +2017     
+ Partials       920       16      -904     

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