fix typing errors

[xHomu]

Fix basic type errors for fetch api.

[ndaidong]

@xHomu thank you for your fix. I'm not familiar with TypeScript, just want to confirm a little bit about Record type at line 134 and 142. Where this type is defined? Or it's one of TS predefined types?

headers?: Record<string, string>;

[xHomu]

Record is a typescript utility type:

https://www.typescriptlang.org/docs/handbook/utility-types.html#recordkeys-type

Record<string, string> generates a type that would accept only objects whose props that's string: string.

[ndaidong]

@xHomu ah thank you, I got it. Now I will merge your pr and resolve CI issue.