fts:match doesn't work across the union graph

Since !131 (merged), SELECT with fts:match requires me to specify a specific graph.

> tracker sparql -q 'SELECT ?u ?text { ?u a nfo:Document ; nie:plainTextContent ?text }'
Results:
  urn:bnode:07c47e7f-07da-41c1-94c3-fbdf301b96e9, Some stupid content, to have a test file
  urn:bnode:290b9217-fa9e-4e12-88f0-74dee1e78153, Some stupid content, to have a test file
  urn:bnode:3d64160d-d725-4851-9276-3d1d36881829, Some stupid content, to have a test file
  urn:bnode:da0f9cbb-8090-4eaa-96b9-6da2feee0c63, automobile is red and big and whatnot

> tracker sparql -q 'SELECT ?u { ?u a nfo:Document ; fts:match "stupid" }'
Results:
  None

> tracker sparql -q 'SELECT ?u FROM <urn:uuid:472ed0cc-40ff-4e37-9c0c-062d78656540> { ?u a nfo:Document ; fts:match "stupid" }'
Results:
  urn:bnode:07c47e7f-07da-41c1-94c3-fbdf301b96e9
  urn:bnode:290b9217-fa9e-4e12-88f0-74dee1e78153
  urn:bnode:3d64160d-d725-4851-9276-3d1d36881829

This is breaking some of the functional tests in tracker-miners.git.