Improve knowledge gap JSON output.

[bnouwt]

The extended SPARQL results which include knowledge-gaps can be improved. The pattern has missing spaces before objects which are variables and maybe the pattern can be printed as a single string instead of a array of strings. The gaps should probably remain an array of strings, because they are not necessary related to each other. 👍

Example output below with domain specific parts left out.

{
	"head": {
		"vars": [
			...
		]
	},
	"results": {
		"bindings": []
	},
	"knowledge_gaps": [
		{
			"pattern": [
				...
			],
			"gaps": [
				[
					...
				]
			]
		}
	]
}