Unit test for invalid HTTPS cert

[dionyziz]

As this is security-related software, I recommend we add a test to check that when connecting to a DNSChain server via HTTPS with an invalid certificate, the connection is rejected client-side with an appropriate error.

May need to mock this properly.

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[zancas]

I'll take this.

[taoeffect]

You'll need this: https://stackoverflow.com/questions/18122843/verify-certificates-fingerprints

[taoeffect]

Also, DNSChain will need to add HTTPS support first (easy to do though, I can do it in a few days, or someone else can do it faster).

[zancas]

I won't commit to adding HTTPS support to DNSChain, but maybe if I'm really productive, I can get around to to it after adding fingerprint verification to the pydnschain node. Anywho, if no-one else's has gotten to it by the time I've polished off the rest of my issues, I'll start on it.