Harden the PathString converter

[HaoK]

Fixes #848 #690

@Tratcher

[davidfowl]

Isn't this in preview2?

[HaoK]

Yeah this is just cleanup, but I can make this change in preview 2 if we want

[Tratcher]

This replicates the asymmitry bug from the implicit converters:
#690

string -> PathString should use FromUriComponent(string), not new PathString(string)

We should change them both at the same time. @davidfowl

[HaoK]

Let me see if I can unify the code a bit to make that change easier

[HaoK]

So I added a new

internal static PathString ConvertFromString(string s) => string.IsNullOrEmpty(s) ? new PathString(s) : FromUriComponent(s);

That both the implicit op and ConvertFrom call, but it wasn't obvious to me exactly how to write a test verifying that the round tripping issue was fixed.

[Tratcher]

The most obvious things that are broken by the asymmetry are the equality operators. E.g. Equals, StartsWithSegments, GetHashCode, etc..
Here's a repro:

            PathString p1 = "/?";
            String s1 = p1; // Becomes "/%3F"
            PathString p2 = s1; // Stays "/%3F"
            Console.WriteLine(p1.Equals(p2)); // false

[HaoK]

Added a test verifying the equality now work

[HaoK]

Now also fixes: #690

[Tratcher]

Stays "/%3F" is wrong after your fix, it gets unescaped again

[Tratcher]

ping

[HaoK]

Yeah I'll merge this later today

[HaoK]

12f89f6