Fixed #2054: can't format heredoc in some cases

spec/compiler/formatter/formatter_spec.cr

@@ -777,4 +777,8 @@ describe Crystal::Formatter do
   assert_format "case 1\nwhen 2 # a\n  # b\nend"
 
   assert_format "{} of A => B\n{} of Foo => Bar"
+
+  assert_format "<<-HTML\n  \#{1}x\n  HTML"
+  assert_format "<<-HTML\n  \#{1}x\n  y\n  HTML"
+  assert_format "<<-HTML\n  \#{1}x\n  y\n  z\n  HTML"
 end

src/compiler/crystal/tools/formatter.cr

@@ -399,6 +399,8 @@ module Crystal
           end
           next_string_token
         else
+          skip_strings
+
           check :INTERPOLATION_START
           write "\#{"
           delimiter_state = @token.delimiter_state
@@ -412,6 +414,8 @@ module Crystal
         end
       end
 
+      skip_strings
+
       check :DELIMITER_END
       write @token.raw
       format_regex_modifiers if is_regex
@@ -420,6 +424,15 @@ module Crystal
       false
     end
 
+    private def skip_strings
+      # Heredocs might indice some spaces that are removed
+      # because of indentation
+      while @token.type == :STRING
+        write @token.raw
+        next_string_token
+      end
+    end
+
     def visit(node : RegexLiteral)
       accept node.value