Metadata overwrite policy can be set via alfresco-global.properties

Steps to reproduce

1. Deploy custom-metadata-extrators-context.xml (attached) to extension folder (which maps PDF producer to cm:taggable)
2. Create folder: folder1
3. Create two rules in folder1 (see screenshot for rules definition)
4. Upload Supported Platforms for Alfresco Enterprise 4.2.x.pdf (attached) into folder1
5. Confirm tag is microsoft® word 2013
6. Trigger action: Upload New Version and upload Supported Platforms for Alfresco Enterprise 5.0.pdf (attached)

Actual result
Tag remains: microsoft® word 2013

Expected result
Tag to be LibreOffice 4.2

Notes

• Also reproducible with the OOTB mappings, title=cm:title
• Image extractor works as expected, that is, image height will be updated on Upload New Version.
• Log shows the correct mapping:

  2015-10-20 12:03:08,955 DEBUG [org.alfresco.repo.content.metadata.AbstractMappingMetadataExtracter] [http-bio-8080-exec-4] Converted extracted raw values to system values: 
   producer=LibreOffice 4.2, Content-Type=application/pdf}
    System Properties: {{http://www.alfresco.org/model/content/1.0}taggable=LibreOffice 4.2}