Define `URI::Generic#as_json`

When the URI object is converted as JSON,
it is expected that it is a string that means its URI.

Expected:
```
>> URI.parse('http://example.com').as_json
"http://example.com"
```

Actual:
```
>> URI.parse('http://example.com').as_json
{"scheme"=>"http",
 "user"=>nil,
 "password"=>nil,
 "host"=>"example.com",
 "port"=>80,
 "path"=>"",
 "query"=>nil,
 "opaque"=>nil,
 "fragment"=>nil,
 "parser"=>
  {"regexp"=>
    {"SCHEME"=>"(?-mix:\\A[A-Za-z][A-Za-z0-9+\\-.]*\\z)",
     "USERINFO"=>"(?-mix:\\A(?:%\\h\\h|[!$&-.0-;=A-Z_a-z~])*\\z)",
     "HOST"=>
      "(?-mix:\\A(?:(?<IP-literal>\\[(?:(?<IPv6address>(?:\\h{1,4}:){6}(?<ls32>\\h{1,4}:\\h{1,4}|(?<IPv4address>(?<dec-octet>[1-9]\\d|1\\d{2}|2[0-4]\\d|25[0-5]|\\d)\\.\\g<dec-octet>\\.\\g<dec-octet>\\.\\g<dec-octet>))|::(?:\\h{1,4}:){5}\\g<ls32>|\\h{,4}::(?:\\h{1,4}:){4}\\g<ls32>|(?:(?:\\h{1,4}:)?\\h{1,4})?::(?:\\h{1,4}:){3}\\g<ls32>|(?:(?:\\h{1,4}:){,2}\\h{1,4})?::(?:\\h{1,4}:){2}\\g<ls32>|(?:(?:\\h{1,4}:){,3}\\h{1,4})?::\\h{1,4}:\\g<ls32>|(?:(?:\\h{1,4}:){,4}\\h{1,4})?::\\g<ls32>|(?:(?:\\h{1,4}:){,5}\\h{1,4})?::\\h{1,4}|(?:(?:\\h{1,4}:){,6}\\h{1,4})?::)|(?<IPvFuture>v\\h+\\.[!$&-.0-;=A-Z_a-z~]+))\\])|\\g<IPv4address>|(?<reg-name>(?:%\\h\\h|[!$&-.0-9;=A-Z_a-z~])*))\\z)",
     "ABS_PATH"=>
      "(?-mix:\\A\\/(?:%\\h\\h|[!$&-.0-;=@-Z_a-z~])*(?:\\/(?:%\\h\\h|[!$&-.0-;=@-Z_a-z~])*)*\\z)",
     "REL_PATH"=>
      "(?-mix:\\A(?:%\\h\\h|[!$&-.0-;=@-Z_a-z~])+(?:\\/(?:%\\h\\h|[!$&-.0-;=@-Z_a-z~])*)*\\z)",
     "QUERY"=>"(?-mix:\\A(?:%\\h\\h|[!$&-.0-;=@-Z_a-z~\\/?])*\\z)",
     "FRAGMENT"=>"(?-mix:\\A(?:%\\h\\h|[!$&-.0-;=@-Z_a-z~\\/?])*\\z)",
     "OPAQUE"=>"(?-mix:\\A(?:[^\\/].*)?\\z)",
     "PORT"=>
      "(?-mix:\\A[\\x09\\x0a\\x0c\\x0d ]*\\d*[\\x09\\x0a\\x0c\\x0d ]*\\z)"}}}
```

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