MIME types that can't be identified as textual are base64 encoded before
being shown in the Network Monitor tab of the developer tools. This text
detection wasn't identifying "custom" JSON MIME-types like
"application/x-bigcorp-api-json" as being JSON, and consequently they were
shown in a rather unhelpful base64 form.
This patch extends and liberalizes the isTextMimeType method to identify
these forms correctly.