Fix problems after Null-Safety Migration and add some improvements

[andresruizdev]

In this pull request, I address the issue found in CurlClient after migrating to Null-Safety and make a series of improvements to the class to support Headers and more request methods using Curl. The main changes made are:

• Explicitly control null values to fix empty values in the args variable after my last commit.
• This PR fixes or closes issue: fixes #13 .
• Add headers support in the curl client.
• Handle disallowed methods according to HTTP/1.1 (RFC 7231)
• Update my old username to the new one in the CHANGELOG.md file.
• Update the version (1.5.2).

[isoos]

published