[whatwg/url] Reparsing problem with non-special URL and double-dot path component (#415)

When the URL `a:/a/..//a` is parsed, the resulting URL object has no hostname, and has the path `//a`

<details>

c = 'a'
state = scheme start state
buffer = ""
pointer = 0

---

c = ':'
state = scheme state
buffer = "a"
pointer = 1

---

scheme = "a"
buffer = ""
state = path or authority state
pointer = 2

---

pointer = 3
c = 'a'
state = path state
pointer = 2

---

state = path state
pointer = 3
c = 'a'
buffer = "a"

---

state = path state
pointer = 4
c = '/'
path = [ "a" ]
buffer = ""

---

state = path state
pointer = 5
c = '.'
path = [ "a" ]
buffer = "."

---

state = path state
pointer = 6
c = '.'
path = [ "a" ]
buffer = ".."

---

state = path state
pointer = 7
c = '/'
path = [""]
buffer = ""

---

state = path state
pointer = 8
c = '/'
path = ["", ""]
buffer = ""

---

state = path state
pointer = 9
c = 'a'
path = ["", ""]
buffer = "a"

---

state = path state
pointer = 10
c = EOF
path = ["", "", "a"]
buffer = ""

</details>

When the resulting URL is serialized, it gets serialized as `a://a`, which, if it gets reparsed, gets an empty path and a hostname `a`.

Found while trying to find a spec-compliant resolution for https://github.com/servo/rust-url/pull/459

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