Fix for #26

[guillaumeserale]

Fix for the following issue:

• OperationHeaders generation problem with headers containing a - #26
  In source\repos\refitter\src\Refitter.Core\ParameterExtractor.cs
  Before:

        if(settings.GenerateOperationHeaders)
        {
            headerParameters = operationModel.Parameters
                .Where(p => p.Kind == OpenApiParameterKind.Header && p.IsHeader)
                .Select(p => $"{JoinAttributes($"Header(\"{p.VariableName}\")", GetAliasAsAttribute(p))}{GetBodyParameterType(p)} {p.VariableName}")
                .ToList();
        }

After:

        if(settings.GenerateOperationHeaders)
        {
            headerParameters = operationModel.Parameters
                .Where(p => p.Kind == OpenApiParameterKind.Header && p.IsHeader)
                .Select(p => $"{JoinAttributes($"Header(\"{p.Name}\")")}{GetBodyParameterType(p)} {p.VariableName}")
                .ToList();
        }

[christianhelle]

This looks good to me! Thanks for the extensive tests as well