Fix adding requests with the security extra (#1439)

Using poetry from master/develop branches or any recent alpha/beta release,
you can't add the requests package with the security extra.
The requests package requires the idna twice
(as a main dependency and as an optional one) and this case wasn't handled
properly in Locker._dump_package function.
These changes make the _dump_package ensures that
all elements of the constraints list have the same type
in order to make them renderable as TOML.

poetry/packages/locker.py

@@ -218,11 +218,6 @@ def _dump_package(self, package):  # type: (poetry.packages.Package) -> dict
 
             if dependency.pretty_name not in dependencies:
                 dependencies[dependency.pretty_name] = []
-            else:
-                # Ensure we don't have mixed types in the lock file
-                for i, spec in enumerate(dependencies[dependency.pretty_name]):
-                    if not isinstance(spec, dict):
-                        dependencies[dependency.pretty_name][i] = {"version": spec}
 
             constraint = {"version": str(dependency.pretty_constraint)}
 
@@ -235,10 +230,15 @@ def _dump_package(self, package):  # type: (poetry.packages.Package) -> dict
             if not dependency.python_constraint.is_any():
                 constraint["python"] = str(dependency.python_constraint)
 
-            if len(constraint) == 1:
-                dependencies[dependency.pretty_name].append(constraint["version"])
-            else:
-                dependencies[dependency.pretty_name].append(constraint)
+            dependencies[dependency.pretty_name].append(constraint)
+
+        # All the constraints should have the same type,
+        # but we want to simplify them if it's possible
+        for dependency, constraints in tuple(dependencies.items()):
+            if all(len(constraint) == 1 for constraint in constraints):
+                dependencies[dependency] = [
+                    constraint["version"] for constraint in constraints
+                ]
 
         data = {
             "name": package.pretty_name,