allow space in SUMMARY.md's link destination

[Evian-Zhang]

Related issues: #527 #1287

As stated in CommonMark Spec:

A link destination consists of either

• a sequence of zero or more characters between an opening < and a closing > that contains no spaces, line breaks, or unescaped < or > characters, or

• a nonempty sequence of characters that does not include ASCII space or control characters, and includes parentheses only if (a) they are backslash-escaped or (b) they are part of a balanced pair of unescaped parentheses. (Implementations may impose limits on parentheses nesting to avoid performance issues, but at least three levels of nesting should be supported.)

and according to this and MDN spec Percent-encoding, there are three ways to allow space in link destination:

- [test1](./test%20link1.md)
- [test2](./test+link2.md)
- [test3](<./test link3.md>)

For now, mdBook only supports method 3, which is rarely used compared to method 1 and 2.

So I fix it and add a test.

[ehuss]

Thanks for the PR! I would prefer to stick with the behavior from markdown where %20 is converted to space. The + transformation isn't something that is normally done for path components in URLs (or in markdown). Would you mind removing the + transformation?

Also, it looks like rustfmt needs to be run.

[Evian-Zhang]

Thanks for the PR! I would prefer to stick with the behavior from markdown where %20 is converted to space. The + transformation isn't something that is normally done for path components in URLs (or in markdown). Would you mind removing the + transformation?

Also, it looks like rustfmt needs to be run.

Ok. I have removed the + transformation and ran rustfmt.

[ehuss]

Thanks!