Add check for undefined headerValue

[mattcollier]

The value of headerValue is not tested before it is passed into _parseHeaderValue here:

https://github.com/digitalbazaar/http-digest-header/blob/master/lib/httpDigest.js#L41

Therefore, it is possible that an unexpected error can occur when calling .split on undefined here:

http-digest-header/lib/httpDigest.js

Lines 65 to 66 in 581fded

	function _parseHeaderValue(headerValue) {
	const [key, encodedDigest] = headerValue.split(/=(.+)/);

There should be a check to ensure that headerValue is a string and if not, an appropriate TypeError should be thrown.

Related: https://github.com/digitalbazaar/bedrock-edv-storage/pull/70/files#r555269144