Double interpolation does not parse correctly

[smpallen99]

~S/%a(href="#{a} #{b}")/ does not parse correctly. Here is the result of a test case.

    ** (CalliopeException) Invalid attribute 'href="1{a' on line number 1`
     stacktrace:
       (calliope) lib/calliope/parser.ex:183: Calliope.Parser.raise_error/3
       (calliope) lib/calliope/parser.ex:140: Calliope.Parser.merge_attributes/2
       (calliope) lib/calliope/parser.ex:38: Calliope.Parser.parse_line/2
       (calliope) lib/calliope/parser.ex:22: Calliope.Parser.parse_lines/1
       (calliope) lib/calliope/parser.ex:18: Calliope.Parser.parse/1
       (calliope) lib/calliope/render.ex:7: Calliope.Render.precompile/1
       test/calliope/render_test.exs:4: CalliopeRenderTest.render/2
       test/calliope/render_test.exs:165: (test)