Fix payload string generation

[ivanpuchalka]

Fixes #10.

To ensure the generated string is a valid Python literal, this new code manually creates an instance of an AST constant node with the value parsed from the HAR and then uses ast.unparse.

A few details worth reviewing:

• The literal generation is implemented as an entriesprocessor_with_args defined in the main module, I don't think it fits as a plugin because it needs to be executed for the program to work properly.

• Since the output now includes the enclosing quotes, I removed them from the default template.

• With the changes, the apple-buy-a-mac test started to fail. It seems the test was wrong: the expected output ignored backslashes that should be in the string, but got interpreted as part of a double quote escape sequence. It's now changed to reflect the correct expected output (and correct behaviour).

[cyberw]

Thanks!

[cyberw]

The tests break when there are changes in the output, they are a bit weird in that way, but it is by design. There’s a make action to update them. I uses the git diff in the generated output to validate that any changes were good :)