This pull request includes several updates to the PostgREST library, focusing on simplifying header and query management, improving JSON serialization, and adding comprehensive test coverage.

Simplification of Header and Query Management:

• Sources/PostgREST/PostgrestBuilder.swift: Simplified the logic for appending or updating the prefer header by introducing the appendOrUpdate method.
• Sources/PostgREST/PostgrestTransformBuilder.swift: Refactored multiple methods to use the appendOrUpdate method for headers and query items, reducing code duplication and improving readability. [1] [2] [3] [4]

JSON Serialization Improvements:

• Sources/PostgREST/URLQueryRepresentable.swift: Modified the stringfy method in JSONSerialization to return an optional String, ensuring safer handling of JSON serialization.

Addition of New Tests:

• Tests/PostgRESTTests/PostgresQueryTests.swift: Added a new test suite PostgrestQueryTests with various tests for the PostgrestClient class, including tests for custom headers, non-success status codes, and different HTTP methods.
• Tests/PostgRESTTests/PostgrestBuilderTests.swift: Extended PostgrestBuilderTests to include tests for custom headers, error handling, and different query options.
• Tests/PostgRESTTests/PostgrestQueryBuilderTests.swift: Introduced PostgrestQueryBuilderTests to test various query builder methods such as select, insert, update, upsert, and delete.