fix: as of parsby 1.1, IO can be mutated during parsing

See 534f8a95b4/lib/parsby.rb (L572C11-L572C17)

Trace:
IOError: not modifiable string
    # parsby-1.1.1/lib/parsby.rb:572:in `ungetc'
    # parsby-1.1.1/lib/parsby.rb:572:in `ungetc'
    # parsby-1.1.1/lib/parsby.rb:539:in `block in restore'
    # parsby-1.1.1/lib/parsby.rb:539:in `each'

app/models/concerns/tags_substitution_concern.rb

@@ -12,7 +12,7 @@ module TagsSubstitutionConcern
     extend self
 
     def parse(io)
-      doc.parse io
+      doc.parse(+io) # parsby mutates the StringIO during parsing!
     end
 
     def self.normalize(str)