Fix double newline after streaming

Check if the streaming command emitted two newlines before adding an extra newline for padding. This fixes the case where an extra newline was being added when it wasn't needed.

libherokubuildpack/src/buildpack_output/mod.rs

@@ -510,18 +510,21 @@ where
     ///
     /// Once you're finished streaming to the output, calling this function
     /// finalizes the stream's output and transitions back to a [`state::Section`].
-    pub fn finish(mut self) -> BuildpackOutput<state::Section<W>> {
+    pub fn finish(self) -> BuildpackOutput<state::Section<W>> {
         let duration = self.state.started.elapsed();
 
-        writeln_now(&mut self.state.write, "");
-
-        BuildpackOutput {
+        let mut output = BuildpackOutput {
             started: self.started,
             state: state::Section {
                 write: self.state.write.unwrap(),
             },
+        };
+
+        if !output.state.write_mut().was_paragraph {
+            writeln_now(&mut output.state.write, "");
         }
-        .step(format!(
+
+        output.step(format!(
             "Done {}",
             style::details(duration_format::human(&duration))
         ))
@@ -701,7 +704,6 @@ mod test {
                   {tab_char}
                   baz
 
-
               - Done (< 0.1s)
             - Done (finished in < 0.1s)
         "};