Adding the option of using POST for queries in SPARQLStore

[SeguinBe]

Solution to #672

Just adding a small parameter to be able to use POST for queries in order to to perform long requests.

Previously :

graph = Graph('SPARQLUpdateStore', identifier='http://localhost:8890/WGA') 
graph.query('<VERY LONG STRING>')
# Query get cropped because it's done with HTTP GET

Now :

from rdflib.plugins.stores.sparqlstore import SPARQLUpdateStore
store = sparqlstore.SPARQLUpdateStore(query_as_post=True)
graph = Graph(store, identifier='http://localhost:8890/WGA') 
graph.query('<VERY LONG STRING>')
# Works!

Tiny changes based on postAsEncoded param in SPARQLUpdateStore

[joernhees]

thanks for the PR... slight change request:
i think this is more "stable" long term... might be interesting to have an automatic failover for too long GETs somewhen...

[joernhees]

could you rename this to default_query_method=GET?

[joernhees]

self.query_method = default_query_method

[joernhees]

self.setMethod(self.query_method)

[SeguinBe]

Yeah having something automatic might be good for users, because the current error message is really not hepful right now (complains about a weird character).

Also, it should be noted that this extension doesn't allow to make a POST query with the sparql query as POST data directly. By default it uses an URL_ENCODED scheme. Not sure if it might be useful at one point though.