Add default headers to constructor

[mielvds]

Adds support for custom headers as proposed in #53. From looking at the existing code, I positioned the change as default headers that get overridden when actually fetching.

I was also wondering whether acceptHeader in https://github.com/rubensworks/fetch-sparql-endpoint.js/blob/master/lib/SparqlEndpointFetcher.ts#L177 should not better generalized to an optional customHeaders object?

[coveralls]

Pull Request Test Coverage Report for Build 5046704468

• 2 of 2 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 100.0%

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Totals
Change from base Build 4464535117:	0.0%
Covered Lines:	85
Relevant Lines:	85

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💛 - Coveralls

[rubensworks]

Thanks @mielvds! :-)

Released as 3.3.0.

I was also wondering whether acceptHeader in https://github.com/rubensworks/fetch-sparql-endpoint.js/blob/master/lib/SparqlEndpointFetcher.ts#L177 should not better generalized to an optional customHeaders object?

There may be some improvements possible there. But since that method is mainly being used internally, this doesn't seem urgent to me.