Embedded quotes in custom attributes break the projection

[asklar]

Describe the bug
Follow up from #818 - quotes in a custom attribute string value will produce invalid C# code

To Reproduce
add a custom attribute like:

[doc_string("the \\\"null\\\" string.\n\n")]
class C { }

this produces the C# code:

doc_string[@"the \"null\" string.\n\n"]
class C { }

which breaks as quotes in @ strings must use double quotes

Expected behavior
double quoted strings

Version Info
1.3.0

Additional context