Check only whether block was given
Check only whether block was given

`rb_scan_args(argc, argv, "01&", ...)` will generate `Proc` object from
block.
However, the object has used to only check whether block was given.

To remove redundant object generating, this patch will use
`rb_block_given_p()` to check whether block was given.

* Before
```
Warming up --------------------------------------
               query   845.000  i/100ms
                each    86.916k i/100ms
              fields   231.527k i/100ms
Calculating -------------------------------------
               query      9.553k (± 2.0%) i/s -     48.320k in
5.059947s
                each      1.133M (± 0.3%) i/s -      5.736M in
5.062606s
              fields      6.319M (± 0.1%) i/s -     31.719M in
5.019960s
```

* After
```
Warming up --------------------------------------
               query   864.000  i/100ms
                each   106.916k i/100ms
              fields   251.255k i/100ms
Calculating -------------------------------------
               query      9.457k (± 3.8%) i/s -     47.520k in
5.032949s
                each      1.550M (± 0.3%) i/s -      7.805M in
5.037029s
              fields      6.233M (± 0.1%) i/s -     31.407M in
5.039049s
```

kamipo/mysql2,brianmario/mysql2,jeremy/mysql2,brianmario/mysql2,kamipo/mysql2,kamipo/mysql2,jeremy/mysql2,sodabrew/mysql2,sodabrew/mysql2,kamipo/mysql2,brianmario/mysql2,sodabrew/mysql2,jeremy/mysql2