Add http-signature-zcap-verify tests to test project

[aljones15]

Each valid unit test should be able to be verified by http-signature-zcap-verify

1. Go through all the valid positive tests.
2. Verify that the result can be passed in the headers to verify and verify successfully.

[aljones15]

This also needs to add non-root zCap tests to the project.

Use: https://github.com/digitalbazaar/bedrock-kms-http/blob/revocation/test/mocha/20-revocation.js

[aljones15]

test coverage in master

  signCapabilityInvocation
    should sign with a(n)
      Ed25519KeyPair
        ✓ a valid root zCap
        ✓ a valid zCap with a capability string
        ✓ a valid zCap with a capability object
        ✓ a valid root zCap with host in the headers
        ✓ a valid root zCap with a capabilityAction
        ✓ a valid root zCap with json
        ✓ a valid root zCap with out json
        ✓ a valid root zCap with digest
        ✓ a root zCap with out a capabilityAction
        ✓ a valid root zCap with UPPERCASE headers
      RSAKeyPair
        ✓ a valid root zCap
        ✓ a valid zCap with a capability string
        ✓ a valid zCap with a capability object
        ✓ a valid root zCap with host in the headers
        ✓ a valid root zCap with a capabilityAction
        ✓ a valid root zCap with json
        ✓ a valid root zCap with out json
        ✓ a valid root zCap with digest
        ✓ a root zCap with out a capabilityAction
        ✓ a valid root zCap with UPPERCASE headers
    should NOT sign with a(n) 
      Ed25519KeyPair
        ✓ a root zCap with out a HTTP method
        ✓ a root zCap with out headers
        ✓ a root zCap with out an invocationSigner
        ✓ a root zCap with out an invocationSigner.sign method
        ✓ a root zCap with out a url and host
        ✓ a zCap if the capability object has no id
      RSAKeyPair
        ✓ a root zCap with out a HTTP method
        ✓ a root zCap with out headers
        ✓ a root zCap with out an invocationSigner
        ✓ a root zCap with out an invocationSigner.sign method
        ✓ a root zCap with out a url and host
        ✓ a zCap if the capability object has no id


  32 passing (3s)


=============================== Coverage summary ===============================
Statements   : 95.65% ( 44/46 )
Branches     : 88% ( 22/25 )
Functions    : 100% ( 2/2 )
Lines        : 95.65% ( 44/46 )
================================================================================

----------|----------|----------|----------|----------|-------------------|
File      |  % Stmts | % Branch |  % Funcs |  % Lines | Uncovered Line #s |
----------|----------|----------|----------|----------|-------------------|
All files |    95.65 |       88 |      100 |    95.65 |                   |
 main.js  |    95.65 |       88 |      100 |    95.65 |            44,117 |
----------|----------|----------|----------|----------|-------------------|

[aljones15]

1. add a test where invocationSigner.sign is not a function
2. add a test where we check that signed.host is deleted when isBrowser is true