third_party: fix strings "true"/"false" in yaml

Strings containing "true" and "false" were converted
to a boolean type when serializing. Fixed.
Example:
type(yaml.decode(yaml.encode('false'))) == string
type(yaml.decode(yaml.encode('true'))) == string

Closes #3476.

test/app-tap/console.test.lua

@@ -21,7 +21,7 @@ local EOL = "\n...\n"
 
 test = tap.test("console")
 
-test:plan(53)
+test:plan(57)
 
 -- Start console and connect to it
 local server = console.listen(CONSOLE_SOCKET)
@@ -67,6 +67,14 @@ test:is(yaml.decode(client:read(EOL))[1], ';', "get delimiter is ';'")
 client:write("require('console').delimiter('');\n")
 test:is(yaml.decode(client:read(EOL)), '', "clear delimiter")
 
+--
+-- gh-3476: yaml.encode encodes 'false' and 'true' incorrectly.
+--
+test:is(type(yaml.decode(yaml.encode('false'))), 'string')
+test:is(type(yaml.decode(yaml.encode('true'))), 'string')
+test:is(type(yaml.decode(yaml.encode({a = 'false'})).a), 'string')
+test:is(type(yaml.decode(yaml.encode({a = 'false'})).a), 'string')
+
 box.cfg{
     listen=IPROTO_SOCKET;
     memtx_memory = 107374182,

third_party/lua-yaml/lyaml.cc

@@ -605,9 +605,12 @@ static int dump_node(struct lua_yaml_dumper *dumper)
       if (utf8_check_printable(str, len)) {
          if (yaml_is_flow_mode(dumper)) {
             style = YAML_SINGLE_QUOTED_SCALAR_STYLE;
-         } else if (strstr(str, "\n\n") != NULL) {
+         } else if (strstr(str, "\n\n") != NULL || strcmp(str, "true") == 0 ||
+		    strcmp(str, "false") == 0) {
             /*
-             * Tarantool-specific: use literal style for string with empty lines.
+             * Tarantool-specific: use literal style for string
+             * with empty lines and strings representing boolean
+             * types.
              * Useful for tutorial().
              */
             style = YAML_LITERAL_SCALAR_STYLE;