`max_path_length` value of 0 in `PkiIssuerSignIntermediateRequest` struct should not be omitted

[adasiunas]

Expected Behavior

When signing intermediate certificate with client.Secrets.PkiIssuerSignIntermediate function with request parameter as:

request := schema.PkiIssuerSignIntermediateRequest{
  ...
  MaxPathLength: 0,  
  ...
}

max_path_length property is omitted from the request body to the vault API, leading to generated certificate without pathlength property:

X509v3 Basic Constraints: critical
                CA:TRUE, pathlen:0 -> missing part 

This might be due to omitempty setting on MaxPathLength int32 json:"max_path_length,omitempty" property in PkiIssuerSignIntermediateRequest struct.

Current Behavior

I think MaxPathLength: 0 should not be omitted and certificate with pathlen:0 should be generated.

Failure Information

v0.4.3

Steps to Reproduce

Simply try to sign intermediate certificate, or build a variable with MaxPathLength = 0 of type PkiIssuerSignIntermediateRequest and json.Encode it. You will notice that json will not contain this value.

Additional Information

I think this property type should be a pointer (*int32). (Or omitempty removed completely).