Improve CONSTRUCT detection

[abrisse]

See https://github.com/ruby-rdf/rdf-sesame/blob/develop/lib/rdf/sesame/repository.rb#L311

Today the detection of SPARQL Construct to define the default format to text/plain is quite ugly (based on regular expression /bconstruct/b). It should be removed or enhance.

[Richard-Degenne]

FWIW, according to the SPARQL specs, the formal definition of a Construct query is that the first word after the prologue is CONSTRUCT.

That shouldn't be that hard to parse properly?