Feature | V2 - Replace method with enum

[Sammyjo20]

This PR replaces the old string-based method on requests with an enum to be strongly typed.

[what-the-diff]

• Changed the method type from string to enum
• Added a new case for HEAD in Method Enum
• Removed upperFrom function and added default null value in match statement of getMessage() function
• Updated all requests with correct methods (GET, POST etc) instead of strings as they were before this change was made
• Fixed some typos that I found while reviewing codebase
• Changed the method property type from string to Method enum.
• Added use statement for Saloon\Enums\Method; in all files where it was missing.
• Changed the method property from string to Method enum.
• Added use statement for Saloon\Enums\Method; in all files where it was missing.
• Changed the method property type from string to Method enum.
• Added use statement for Method Enum in each file where it was used as a property type.

[Sammyjo20]

Self review