[Fix] fix for an impossible situation: when the formatter is called with a non-string value

Note that all these tests passed already. Since the only time a
formatter is called is in a context where it is concatenated with
another string using `+`, this is a redundant step. However, for
pedantic correctness and documentation, the contract for formatters is
to always return a string.

Author: ljharb

lib/formats.js

@@ -10,7 +10,7 @@ module.exports = {
             return replace.call(value, percentTwenties, '+');
         },
         RFC3986: function (value) {
-            return value;
+            return String(value);
         }
     },
     RFC1738: 'RFC1738',

test/stringify.js

@@ -469,6 +469,12 @@ test('stringify()', function (t) {
                 return String.fromCharCode(buffer.readUInt8(0) + 97);
             }
         }), 'a=b');
+
+        st.equal(qs.stringify({ a: SaferBuffer.from('a b') }, {
+            encoder: function (buffer) {
+                return buffer;
+            }
+        }), 'a=a b');
         st.end();
     });
 
@@ -509,17 +515,20 @@ test('stringify()', function (t) {
     t.test('RFC 1738 spaces serialization', function (st) {
         st.equal(qs.stringify({ a: 'b c' }, { format: qs.formats.RFC1738 }), 'a=b+c');
         st.equal(qs.stringify({ 'a b': 'c d' }, { format: qs.formats.RFC1738 }), 'a+b=c+d');
+        st.equal(qs.stringify({ 'a b': SaferBuffer.from('a b') }, { format: qs.formats.RFC1738 }), 'a+b=a+b');
         st.end();
     });
 
     t.test('RFC 3986 spaces serialization', function (st) {
         st.equal(qs.stringify({ a: 'b c' }, { format: qs.formats.RFC3986 }), 'a=b%20c');
         st.equal(qs.stringify({ 'a b': 'c d' }, { format: qs.formats.RFC3986 }), 'a%20b=c%20d');
+        st.equal(qs.stringify({ 'a b': SaferBuffer.from('a b') }, { format: qs.formats.RFC3986 }), 'a%20b=a%20b');
         st.end();
     });
 
     t.test('Backward compatibility to RFC 3986', function (st) {
         st.equal(qs.stringify({ a: 'b c' }), 'a=b%20c');
+        st.equal(qs.stringify({ 'a b': SaferBuffer.from('a b') }), 'a%20b=a%20b');
         st.end();
     });