Fix randomly-failing CMIS test testDeleteContentStreamPWC

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ERROR: testDeleteContentStreamPWC (cmislibtest.DocumentTest)
Delete the content stream of a PWC
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Traceback (most recent call last):
  File "/opt/jenkins/workspace/master/FT-nuxeo-master-cmis-cap-tomcat-multidb-linux/Slave/MULTIDB_LINUX/dbprofile/mssql/jdk/java-8-oracle/nuxeo-distribution/nuxeo-server-cmis-tests/cmislibtest.py", line 1271, in testDeleteContentStreamPWC
    pwc.deleteContentStream()
  File "/opt/jenkins/workspace/master/FT-nuxeo-master-cmis-cap-tomcat-multidb-linux/Slave/MULTIDB_LINUX/dbprofile/mssql/jdk/java-8-oracle/nuxeo-distribution/nuxeo-server-cmis-tests/cmislib/atompub/binding.py", line 2602, in deleteContentStream
    **args)
  File "/opt/jenkins/workspace/master/FT-nuxeo-master-cmis-cap-tomcat-multidb-linux/Slave/MULTIDB_LINUX/dbprofile/mssql/jdk/java-8-oracle/nuxeo-distribution/nuxeo-server-cmis-tests/cmislib/atompub/binding.py", line 150, in delete
    self._processCommonErrors(resp, url)
  File "/opt/jenkins/workspace/master/FT-nuxeo-master-cmis-cap-tomcat-multidb-linux/Slave/MULTIDB_LINUX/dbprofile/mssql/jdk/java-8-oracle/nuxeo-distribution/nuxeo-server-cmis-tests/cmislib/cmis_services.py", line 62, in _processCommonErrors
    raise UpdateConflictException(error['status'], url)
UpdateConflictException: Error 409 at http://localhost:8080/nuxeo/atom/cmis/default/content/250px-Cmis_logo.png?id=696aec77-4fc4-4c64-b499-53f208384985
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ERROR: testSetContentStreamPWCMimeType (cmislibtest.DocumentTest)
Check the mimetype after the PWC checkin
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Traceback (most recent call last):
  File "/opt/jenkins/workspace/master/FT-nuxeo-master-cmis-cap-tomcat-multidb-linux/Slave/MULTIDB_LINUX/dbprofile/mssql/jdk/java-8-oracle/nuxeo-distribution/nuxeo-server-cmis-tests/cmislibtest.py", line 1192, in testSetContentStreamPWCMimeType
    pwc.setContentStream(f)
  File "/opt/jenkins/workspace/master/FT-nuxeo-master-cmis-cap-tomcat-multidb-linux/Slave/MULTIDB_LINUX/dbprofile/mssql/jdk/java-8-oracle/nuxeo-distribution/nuxeo-server-cmis-tests/cmislib/atompub/binding.py", line 2565, in setContentStream
    **args)
  File "/opt/jenkins/workspace/master/FT-nuxeo-master-cmis-cap-tomcat-multidb-linux/Slave/MULTIDB_LINUX/dbprofile/mssql/jdk/java-8-oracle/nuxeo-distribution/nuxeo-server-cmis-tests/cmislib/atompub/binding.py", line 212, in put
    self._processCommonErrors(resp, url)
  File "/opt/jenkins/workspace/master/FT-nuxeo-master-cmis-cap-tomcat-multidb-linux/Slave/MULTIDB_LINUX/dbprofile/mssql/jdk/java-8-oracle/nuxeo-distribution/nuxeo-server-cmis-tests/cmislib/cmis_services.py", line 62, in _processCommonErrors
    raise UpdateConflictException(error['status'], url)
UpdateConflictException: Error 409 at http://localhost:8080/nuxeo/atom/cmis/default/content/250px-Cmis_logo.png?id=106c17ef-85aa-46cd-8cee-31dbb0313ad9