Reserved keywords cause error when used in an expression with spaces around the keyword

[taylorzane]

When using reserved JavaScript keywords in an expression, if the expression has spaces in between the curly brackets and the keyword, the parser will throw an error.

This throws: <div class="{ class }"></div>
But this does not: <div class="{class}"></div>

This goes for all JS keywords. I've tested import, export, and class at least.

REPL with failing test case: https://svelte.technology/repl?version=2.5.0&gist=9d4b8a0c733a48397d8a66feb9881fa5

REPL with passing test case: https://svelte.technology/repl?version=2.5.0&gist=6149f93a9af8cbc6286a335b8afbfaec

[Conduitry]

I haven't tested this but I think we should be able to fix this with just a parser.allowWhitespace() inserted as the first line of the readExpression function here and have it not break sourcemaps.

[Conduitry]

Actually, this is currently working with things like { export } appearing in HTML, just not when they're in an attribute value. These both use readExpression, so that suggests maybe the allowWhitespace() ought to be inserted elsewhere, but I'm not sure where.

[Conduitry]

Ah, right here.

[taylorzane]

How swift! Thank you for this.

[Rich-Harris]

released 2.5.1 with the fix — thanks