set default protocol to tcp

### Issue description
Can tcp of the connection string be set as a default protocol option, which will fix the url standard, we can easily parse out the parameters of the entire connection string, and adding the tcp parameter will cause the entire string to fail to complete according to the url standard Parsing.

### Example code
```go
“mysql://root:123456@127.0.0.1:3339/ad_base?charset=utf8”
```
this url can be parsed.but this url can not be parsed because of tcp():
```go
"mysql://root:123456@tcp(127.0.0.1:3339)/ad_base?charset=utf8"
```

### Configuration
*Driver version (or git SHA):latest

*Go version:go 1.12-1.14

*Server version: MySQL 8.0

*Server OS:* E.g. Mac OS Catalina