text: Title: [BUG] Time index name is lost after using prepend_values
  Body: **Describe the bug**
  The name of the time_index changes to 'time' after using `prepend_values`.
  
  **To Reproduce**
  ```python
  import numpy as np
  import pandas as pd
  from darts import TimeSeries
  from darts.utils.timeseries_generation import generate_index
  
  start = pd.Timestamp("2000-01-01")
  end = pd.Timestamp("2000-12-31")
  freq = pd.Timedelta(weeks=1)
  
  index = generate_index(start=start, end=end, freq=freq, name='date')
  values = np.random.normal(0, 1, size=len(index))
  
  ts = TimeSeries.from_times_and_values(index, values, freq=freq, columns=pd.Index(['value']))
  ts2 = ts.prepend_values(np.repeat(0, 30))
  
  ts.time_index.name, ts2.time_index.name # ('date', 'time')
  ```
  
  **Expected behavior**
  The name of the time_index should stay the same when values are prepended.
  
  