corsMode calculation in the "Processing Model" section is broken

[tyoshino]

/cc @yutakahirano

https://w3c.github.io/beacon/#sec-processing-model

corsMode is declared in one of the substeps in the step 4 but referenced from the step 5. Instead, it should be declared at the top level and modified by the step 4.

I also think the step 5 should be part of the step 4.

headerList should be initialized as an empty header list, not null.

So, it should be:

4. Let fetchMode be "no-cors".
5. Let headerList be a new empty header list.
6. If data is not null:
   • Extract ...
   • If the amount ...
   • If mimeType is not null:
     • If Content-Type/mimeType is not a CORS-safelisted request-header, set fetchMode to "cors".
     • Append Content-Type/mimeType to headerList.

[igrigorik]

@tyoshino thanks, good suggestions! I opened #54 to address this -- ptal.