properly emit 0 in compressed mode

src/value/number/mod.rs

@@ -444,15 +444,24 @@ impl Number {
             }
         }
 
-        if self.is_negative() && (!whole.is_zero() || !dec.is_empty()) {
+        let has_decimal = !dec.is_empty();
+
+        if self.is_negative() && (!whole.is_zero() || has_decimal) {
             buf.push('-');
         }
 
-        if !(whole.is_zero() && is_compressed) {
+        // if the entire number is just zero, we always want to emit it
+        if whole.is_zero() && !has_decimal {
+            return "0".to_owned();
+
+        // otherwise, if the number is not 0, or the number before the decimal
+        // _is_ 0 and we aren't in compressed mode, emit the number before the
+        // decimal
+        } else if !(whole.is_zero() && is_compressed) {
             buf.push_str(&whole.to_string());
         }
 
-        if !dec.is_empty() {
+        if has_decimal {
             buf.push('.');
             buf.push_str(&dec);
         }

tests/compressed.rs

@@ -116,3 +116,9 @@ test!(
     "a{color:grayscale(0.5)}",
     grass::Options::default().style(grass::OutputStyle::Compressed)
 );
+test!(
+    retains_zero_without_decimal,
+    "a {\n  color: 0.0;\n  color: 0;\n}\n",
+    "a{color:0;color:0}",
+    grass::Options::default().style(grass::OutputStyle::Compressed)
+);