all: return error from concrete type for non-UTF8 strings

[vangent]

Applied to the same set of strings we're escaping:

• blob key
• blob metadata key & value
• pubsub metdata key & value

Verified with conformance tests.

Fixes #1322.

[jba]

What about keys passed elsewhere (attributes, NewRangeReader, etc.)?

[vangent]

What about keys passed elsewhere (attributes, NewRangeReader, etc.)?

For attributes, I haven't added any escaping for those yet. I'm not that worried about it since I don't think there's any expectation that user-invented strings should work (e.g., the reasonable values are defined by some spec). If it becomes a problem we can add validation of those fields, but it's likely to be much more than just "is UTF8".

For NewRangeReader, I expect that they are going to get some kind of error anyway since there's no way the blob exists with a non-UTF8-string key. Do you think it's worth adding a check (and also in Delete, etc.) to short-circuit?

[jba]

I expect that they are going to get some kind of error anyway since there's no way the blob exists with a non-UTF8-string key.

Is that true across all providers? None of them support binary key names?

[vangent]

I expect that they are going to get some kind of error anyway since there's no way the blob exists with a non-UTF8-string key.

Is that true across all providers? None of them support binary key names?

Pretty sure. Well, memblob probably does. It's probably safer to verify, so I've added that.

[vangent]

PTAL