Add tdhttp.Path() function about go-testdeep

[maxatome]

After some thought, it could be better to simply add a tdhttp.Q type:

type Q map[string]interface{}

Instances of this type can be directly used in net/http request builders, like tdhttp.NewRequest as in:

req := tdhttp.NewRequest("POST", "/pdf", body,
  "Content-type", "application/pdf",
  tdhttp.Q{
    "param1": 128,
    "param2": "value"
    "param3": []int{1, 2, 3},
    "param4": anyType, // passed to fmt.Sprint
  },
  "X-Test", "value",
  tdhttp.Q{
    "param3": "another", merged with previous "param3"
    "param5": 45.63,
  },
)