BUG: Unsafe type assertion without ok check in fetcher

[andrinoff]

Describe the bug
Type assertion at fetcher/fetcher.go:883 on p.Header without ok check. Malformed MIME headers cause panic.

To reproduce

1. Receive email with malformed MIME structure
2. Type assertion fails
3. Application panics

Expected behavior
Use type assertion with ok check: h, ok := p.Header.(*mail.InlineHeader)

Matcha version
master

OS
All

Additional context
File: fetcher/fetcher.go line 883
Panic on unexpected header type