Add compatibility with ownCloud 9.1

[joneug]

To add compatibility with ownCloud 9.1 the class AbstractBearer was imported from sabre/dav 3.2. This was necessary, because in ownCloud 9.1 an older version of this library is being used.

This Pull Request can only be built successfully if Pull Request core#27370 is going to be merged.

[mention-bot]

@joneug, thanks for your PR! By analyzing the history of the files in this pull request, we identified @NinaHerrmann to be a potential reviewer.

[codecov]

Codecov Report

Merging #28 into master will increase coverage by 0.11%.
The diff coverage is 100%.

@@             Coverage Diff              @@
##             master      #28      +/-   ##
============================================
+ Coverage     96.83%   96.95%   +0.11%     
- Complexity      143      148       +5     
============================================
  Files            17       18       +1     
  Lines           474      492      +18     
============================================
+ Hits            459      477      +18     
  Misses           15       15

Impacted Files	Coverage Δ	Complexity Δ
lib/Sabre/OAuth2.php	100% <ø> (ø)	7 <0> (ø)	⬇️
lib/Sabre/AbstractBearer.php	100% <100%> (ø)	5 <5> (?)

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