Fix non-merging matching contexts

[chuwy]

Having two items of same context, but with different revision/addition they result in two distinct keys in JSON output object:

Input:

[
  iglu:org.acme/context_one/jsonschema/1-0-0, 
  iglu:org.acme/context_one/jsonschema/1-0-1
]

Output:

{
  "contexts_org_acme_context_one_1" : [ {
    "item" : 1
  } ],
  "contexts_org_acme_context_one_1" : [ {
    "item" : 2
  } ]
}

Works fine for equal schema versions.

Scheduling for urgent 0.2.1 bugfix release.

[alexanderdean]

AHA!