Prevent panic when calling `url::Url::set_host` with `None`.

[frewsxcv]

Fixes #243.

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This change is 

[frewsxcv]

r? @SimonSapin

[valenting]

Note this line from the URL spec:

If url is special and buffer is the empty string, return failure.

So both set_host(None) and set_host("") should probably fail if SchemeType::from(self.scheme()).is_special()

So while the test should not crash, the result should not be https:/hello
The same test would be correct though, if the protocol were not special.

[frewsxcv]

@SimonSapin Do you remember why there is logic here for the 'cannot-be-a-base-URL' flag? At least for the link that @valenting sent, it doesn't look like that logic needs to be here.

[SimonSapin]

@bors-servo r+

[bors-servo]

📌 Commit 2c9e076 has been approved by SimonSapin

[bors-servo]

⌛ Testing commit 2c9e076 with merge ae7204f...

[bors-servo]

☀️ Test successful - status-travis