Return 409 when trying to PUT a file into a non-existent collection

[phil-davis]

Fixes #1304

[codecov]

Codecov Report

Merging #1306 (f02549c) into master (8c15d19) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##             master    #1306   +/-   ##
=========================================
  Coverage     97.14%   97.14%           
- Complexity     2792     2793    +1     
=========================================
  Files           174      174           
  Lines          8117     8119    +2     
=========================================
+ Hits           7885     7887    +2     
  Misses          232      232           

Impacted Files	Coverage Δ	Complexity Δ
lib/DAV/Server.php	96.15% <100.00%> (+0.01%)	193.00 <0.00> (+1.00)

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[phil-davis]

Travis CI has passed: https://travis-ci.org/github/sabre-io/dav/builds/742368329
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[staabm]

Lgtm