Unquoting header keys and values **after** calling `str.strip()` opened
the possibility for user encoding errors where a header contained
invalid whitespace. Unquoting **before** `str.strip()` aligns
`opentelemetry-python` with other `opentelemetry-{lang}` libraries that
implemenent URL decoding.

The HTTP exporter allows headers to be provided as `Dict[str, str]`.
The gRPC exporter required headers to be `Sequence[Tuple[str, str]]`.

This commit enables the `Dict[str, str]` format for gRPC.